Hydrogen Atom Abstraction by Metal−Oxo and Metal−Superoxo Complexes:  Kinetics and Thermodynamics

The superoxochromium complex Cr aq OO 2+ abstracts a hydrogen atom from CMe 3 CHO in acidic aqueous solution with k = 0.16 M - 1 s - 1 . This rate constant is only ∼10 2 times smaller than that for the reaction of Cr aq O 2+ with the same aldehyde, k = 23 M - 1 s - 1, in contrast to the much greater reactivity difference between alkoxyl and alkylperoxyl radicals, k t - BuO / k t - BuOO ≈ 10 6 . The absolute rate constants for hydrogen atom abstraction from a common reagent by metal-oxo and -superoxo species and the corresponding organic oxygen-centered radicals, RO • and ROO •, can now be compared for the first time: k BuO (9 × 10 7 M - 1 s - 1 ) > k CrO (23) ≥ k BuOO (8) > k CrOO (0.16). The reactivity of individual species is explained by the energetics of the O−H bonds in ROH, ROOH, Cr aq OH 2+, and Cr aq OOH 2+ .